The present invention relates to peel ply materials, and in particular to a peel ply material of high visual discernibility for use during curing of a resin coated substrate.
In constructing an article, such as the wing surface of a F-18 Hornet fighter jet airplane, one may wish to use a multi-layer laminate (for example, a 24 layer laminate) having alternating layers of a thermosetting or thermoplastic resin and a graphite or fiberglass substrate. The resin is typically an epoxy or polyimide. Curing is typically done by a combination of heat and pressure over time--for example, about 350.degree. F. and 300 psi over 24 hours. During laying down of the layers of the laminate, air becomes entrapped and, during the curing process, this entrapped air, as well as any gases evolved during the curing reaction, must be purged. If all 24 layers were cured at once, it would be impossible for the gas to escape from the lower layers through the upper layers. Accordingly, a laminate of fewer layers (typically four layers) is first individually formed and cured to start a stack of cured laminates; thereafter successive layers of laminates are placed one on top of the other (four at a time) and cured (four at a time) in turn to increment the stack until it becomes a cured 24 layer laminate.
It is well recognized that successive layers of laminates will not stack and bind properly to the cured stack unless the top surface of the cured stack is sufficiently rough (that is, not smooth). In order to create this roughness, during curing of the four layer laminate a peel ply material in the form of a textured fabric is typically applied over the top of the upper layer; after curing of the four layer laminate, the peel ply material is peeled off and thrown away to reveal the rough or textured upper surface of the laminate stack which will then bind properly with the bottom of the next four layers of laminate placed on top. The peel ply material is typically nylon or polyester.
It will be appreciated that even where the entire stack of laminates is being cured simultaneously, a peel ply layer is still required on top to enable subsequent surface treatment such as an application of paint, a radar- or lightning-absorbing coating, etc.
Various layers of porous material (e.g., breather and ventilating plies and a caul plate) are placed over the peel ply, these layers of porous material being covered with a non-porous vacuum sealing material called a bag. The combination of pressure on top of the sealing material, vacuum below the sealing material, and heat to the stack effects curing. After curing, the peel ply, porous and sealing layers are removed.
In practice, however, the semi-skilled labor overseeing the lamination process frequently forget to remove the peel ply material. This is understandable because the peel ply material is typically white to begin with and "wets" or becomes transparent (and therefore assumes the color of the resin from the laminate) so that it is hard to detect on the cured resin. The presence of the peel ply material in the final 24 layer laminate severely weakens the structure physically since there is a very poor bond between the peel ply material and the resin. At this stage if the peel ply material is discovered within the finished laminate, the finished laminate (which may be worth several million dollars) must be discarded.
A peel ply material must not only be inert to the chemicals (such as the resin), temperatures and pressures involved in the laminating or curing process, but also have a zero transfer level to the resin-coated substrate (i.e., have no ingredients which separate from the peel ply material and otherwise contaminate the laminate).
One proposed solution to the problem is to color the peel ply material with dyes contrasting with the resin, either overall or in patterns--e.g., with printing saying "remove me!". The problem is that the coloring dyes tend to come off or transfer onto the resin and themselves become contaminants as bad as trapped gas. The obvious solution to the contamination problem--to print only on the upper face of the peel ply material so that the lower face contacting the resin could not contaminate it--does not work; unfortunately, the same people who frequently forget to remove the peel ply material from the stack will occasionally apply the wrong surface of the peel ply material to the resin, thus resulting at least in print contamination of the stack and possibly failure to remove the peel play layer as well. Another proposed solution is to not color the entire material, but only employ trace threads (very thin, widely spaced apart stripes or grids of dyed fibers) so there is very little dye to contaminate the laminate; actually in most instances, but not all, this works fairly well if a moderate transfer level is acceptable for the particular application intended. A theoretical solution is to use tracer threads composed of a material of a naturally contrasting color which will not contaminate the laminate because it has no dyes; the problem with this solution is that the known suitable trace threads have different heat shrinkage properties than the materials used in the laminates, thereby resulting in premature separation of the peel ply layer from the remainder of the laminate during curing, deformation of the surface texture, and/or interference in the controlled outgassing process for curing.
Thus, a need remains for a peel ply material of high visual discernibility which combines a base fabric characterized by a high level of wet-out when disposed on the cured resin-coated substrate and tracers exhibiting a low level of wet-out, substantially the same shrinkage rate as the base fabric, and a substantially zero contamination transfer level to the resin-coated substrate. To reduce labor costs, the peel ply material should also be sufficiently economical that it may be used once and discarded.
Accordingly, it is an object of the present invention to provide a peel ply material of high visual discernibility comprising a base fabric characterized by a high level of wet-out and tracers, woven into the base fabric, characterized by a low level of wet-out.
Another object is to provide such a peel ply material wherein the tracers exhibit substantially the same heat shrinkage properties as the base fabric and a substantially zero transfer level to the resin-coated substrate.
A further object is to provide such a peel ply material which is sufficiently economical for one-time use only.
It is also an object to provide such a peel ply material which is inert to the chemicals, temperatures and pressures used in the curing process.